Apparatus and method for forming an oxynitride insulating layer on a semiconductor wafer

ABSTRACT

An apparatus and method of forming an oxynitride insulating layer on a substrate performed by putting the substrate at a first temperature within the main chamber of a furnace, exposing the substrate to a nitrogen containing gas at a second temperature which is higher than the first temperature, and growing the oxynitride layer on the substrate within the main chamber in the presence of post-combusted gases. The higher temperature nitrogen containing gases are combusted in a chamber outside the main chamber. The higher temperature is in the range of 800 to 1200° C., and preferably 950° C. In a second embodiment, distributed N 2 O gas injectors within the main chamber deliver the nitrogen containing gas. The nitrogen containing gas is pre-heated outside the chamber. The nitrogen containing gas is then delivered to a gas manifold that splits the gas flow and directs the gas to a number of gas injectors, preferably two to four injectors within the main process tube. Gas injection orifices on the order of several millimeters then distribute the pre-decomposed gas to the wafers, producing a more uniformly N-doped wafer load in a batch furnace.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the fabrication ofsemiconductor wafers. More particularly, the invention relates to anapparatus and method for employing nitrogen oxide (NO) or N₂O gases todope silicon dioxide in a furnace at high temperatures to form oxidesand oxynitrides layers on a semiconductor for gate dielectrics.

2. Description of Related Art

The ability to use silicon dioxide (SiO₂) as the gate dielectricmaterial becomes extremely difficult for thickness (t_(ox)) less thantwenty (20) angstroms (Å). Thickness as small as 20 angstroms arerequired for device scaling with channel lengths less than 0.25 microns(μm). For thickness less than twenty angstroms, leakage currents mayapproach 1 A/cm². This is significant when compared to leakages on theorder of 1(10⁻¹²) A/cm² for thickness t_(ox) greater than 40 angstroms.Thus having a thickness on the order of twenty angstroms may produceprohibitive power consumption of the transistors in the off-state, andreliability concerns through lifetime degradation, i.e., devicelifetimes less than ten (10) years.

The leakage current is caused by direct tunneling of electrons from thepolysilicon gate electrode through the gate oxide. Boron from the dopedpoly-silicon gate easily penetrates the thin SiO₂ layer causing largeV_(t) shifts and more reliability problems. Boron doped poly-silicongate electrodes are required to avoid depletion effects which will alsocause large V_(t) shifts and higher threshold voltages. Siliconoxynitrides (SiOxNy) or nitrogen (N) doped SiO₂ have been chosen by mostintegrated circuit chip manufacturers as the material of choice toreplace SiO₂ for gate dielectrics in the thickness range of 15 to 20angstroms. The beneficial effects of nitrogen incorporation aredependent upon the magnitude of the doping and the distribution of thedoping profile relative to both the Si/SiO₂ interface and thepoly-silicon gate/SiO₂ interface. If the nitrogen doping is engineeredcorrectly, leakage current and boron penetration can be reduced, whileminimizing or negating the impact on threshold voltage V_(t) and channelelectron mobility. Additionally, hot-electron defect generation in thesilicon channel can be reduced by nitrogen gettering of hydrogen (H).These effects make scaling the gate dielectric down to fifteen (15)angstroms viable, while minimizing the impact on process integrationthat would occur by changing the gate dielectric to a high dielectricconstant (K) material system; the high-K material being differentmaterial than the SiO₂. The ability to correctly engineer the nitrogendopant profile is absent in the prior art.

Oxides and oxynitrides for gate dielectrics are typically grown inatmospheric (or reduced pressure) furnaces, where the gas ispre-combusted through a torch injector pre-tube outside of the mainprocess tube. The resultant product is then delivered to the mainprocess tube for reaction with wafers that are pre-processed up to thegate dielectric layer. Typical torch combustion chambers are engineeredto preheat gas up to 850° C. for combustion of O₂ and H₂ to form H₂O andO₂. H₂O is a critical reactant for wet oxidation in the formation ofhigh quality gate oxides, and has been used extensively as such by thesemiconductor industry. The torch is also used to combust chlorinecontaining sources to provide high purity atomic chlorine that is usedas a metal getterer in the furnace process chamber which, along withnitrogen oxide (NO), are the two gases that can be used to thermallygrow or anneal high quality oxynitride films. However, torches have notbeen engineered for N₂O combustion. For example, in U.S. Pat. No.6,017,791, issued to Wang, et al., entitled, “MULTI-LAYER SILICONNITRIDE DEPOSITION METHOD FOR FORMING LOW OXIDATION TEMPERATURETHERMALLY OXIDIZED SILICON NITRIDE/SILICON OXIDE (NO) LAYER,” a methodfor forming a silicon nitride/silicon oxide (NO) layer within amicroelectronics fabrication was introduced. In order to form theselayers, Wang uses two deposition methods within the same depositionreactor chamber, with the first deposition method separated from thesecond deposition method by a vacuum purge of the deposition reactorchamber to assure that the second silicon nitride layer is formed as adiscrete silicon nitride layer upon the first silicon nitride layer.Each of the nitride layers in the Wang art are formed through a chemicalvapor deposition (CVD) process. By implementing a higher temperatureoutside the main process tube (chamber), the instant inventioneliminates, among other things, the vacuum purge of the depositionreactor chamber.

Nitrogen oxide is not typically used since it requires additional safetyapparatus due to its toxicity. It can also produce certain undesirableelectrical properties of the device, such as low electron channelmobility, large voltage threshold shifts, and hot-electron degradationeffects.

The present invention introduces a thermal nitrogen dopant tuner and itsability to make use of the N₂O decomposition mechanisms to fabricatetransistors with thickness in the ten to twenty angstrom regime.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide an apparatus andmethod for utilizing N₂O decomposition mechanisms to make transistorswith thickness as small as ten to twenty angstroms.

It is another object of the present invention to provide an apparatusand method for using SiO₂ as a gate dielectric material for thickness onthe order of 10-20 Å.

A further object of the invention is to provide an apparatus and methodfor having a substrate with a gate dielectric thickness less than 20 Åwithout producing prohibitive power consumption due to leakage currentlosses.

It is yet another object of the present invention to provide anapparatus and method for developing nitrogen doped SiO₂ substrate layersto replace SiO₂ layers as gate dielectrics with thickness on the orderof 10-20 Å.

Another object of the invention is to provide an apparatus and methodfor reducing leakage current and boron penetration of the SiO₂ layerwhile negating the impact on threshold voltage and channel electronmobility.

A further object of the invention is to provide an apparatus and methodfor scaling gate dielectrics down to the 10-20 Å regime while minimizingthe impact on process integration.

Still other advantages of the invention will in part be obvious and willin part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other advantages, which will be apparent to one of skillin the art, are achieved in the present invention which is directed to,in a first aspect, a method of forming an insulating layer on asubstrate comprising pre-combustion of nitrogen containing gas outside afurnace having a main chamber, the pre-combustion performed at atemperature higher than that within the main chamber, wherein thenitrogen containing gas includes N₂O or NO.

In a second aspect, the instant invention is directed to a method oftuning the magnitude of nitrogen doped profiles for an oxynitrideinsulator on a substrate, the method comprising: providing the substrateat a first temperature within a main chamber; exposing the substrate toa nitrogen containing gas at a second temperature which is higher thanthe first temperature; and, growing the insulator on the substrate inthe presence of the gas. The exposing step further comprises heating thenitrogen containing gas to the second temperature in a second chamberoutside the main chamber and directing the heated nitrogen containinggas to the main chamber. The method further comprises heating chlorineand steam gas to a combustion temperature and transferring the chlorineand steam gas to the second chamber. The heating is accomplished byapplying a torch heating element to the chlorine and steam gas in athird chamber separate from the main and second chambers. The secondtemperature is in the range of 800 to 1200° C., preferably 950° C. Thefirst temperature is in the range 600 to 1100° C., or preferably in therange 600 to 800° C. The second temperature is sufficient to react withthe gas before it reacts with the substrate. The second temperature isadjusted to tailor an amount of nitrogen concentration in the insulatinglayer.

In a third aspect, the instant invention is directed to a method foremploying a nitrogen containing gas to form an insulation film on asemiconductor, comprising:

-   -   combusting chlorine and steam gas in a first chamber,        transporting the combusted chlorine and steam gas to a second        chamber; applying heat to the second chamber to the chlorine and        steam gas, removing the chlorine and steam gas from the second        chamber to a third chamber, and introducing nitrogen containing        gas to the second chamber from the first chamber, applying heat        to the second chamber to the nitrogen containing gas;        transferring the nitrogen containing gas to a third chamber        containing the semiconductor, and, applying heat to the third        chamber at a temperature level sufficient to initiate a        gas-semiconductor reaction wheren the insulation film is formed        on the semiconductor.

In a fourth aspect, the instant invention is directed to a method fordistributing a nitrogen containing gas to form an insulation film on asemiconductor, comprising: providing the semiconductor within a mainprocess tube; pre-heating N₂O gas outside the main process tube;delivering the N₂O gas to a gas manifold; splitting the N₂O gas flowthrough the gas manifold to direct the N₂O gas to gas injectors attachedto the main process tube; and, distributing the N₂O gas through theinjectors to the main process tube housing the semiconductor.

In a fifth aspect, the instant invention is directed to an apparatus forforming an insulating layer on a substrate comprising: a furnaceincluding: a first chamber having a first heating element for combustingchlorine and steam gas at a first temperature; a second chamber having asecond heating element, the second chamber adapted to receive thechlorine and steam gas and separately a nitrogen containing gas at asecond temperature; and, a third chamber having a third heating elementat a third temperature adapted to react separately the gases from thesecond chamber with the substrate to grow the insulating layer. Thesecond temperature is higher than the third temperature, such that thecombined gases from the second chamber enter the third chamber at ahigher temperature than the third temperature. The first heating elementcomprises a torch. The second heating element may also be comprised of atorch.

In a sixth aspect, the instant invention is directed to an apparatus fordistributing a nitrogen containing gas to form an insulation film on asemiconductor, comprising: a furnace main process tube adapted for gasinjection within the tube and securing the semiconductor; a heatingelement outside the main process tube for preheating a nitrogencontaining gas; a gas manifold adapted to receive and deliver thepre-heated nitrogen containing gas to gas injectors attached to the mainprocess tube; the gas injectors adapted to distribute the nitrogencontaining gas to the semiconductor. The gas injectors include at leasttwo injectors within the main process tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a graph of a comparison of capacitance-voltage curves for athermal nitrogen dopant tuner process and a process using SiO₂ control.

FIG. 2 represents I-V curves for the thermal nitrogen dopant tuner filmand the SiO₂ control.

FIG. 3 is a block diagram representing the gas flow through the separatechambers used in the gas flow/reaction sequence of the instantinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-3 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

A torch or pre-combustion furnace chamber that is engineered for hightemperature combustion is used to tune the magnitude and position of thenitrogen doping profile for oxynitrides. Tuning of the nitrogen profilein an oxynitride gate dielectric can lower the leakage current and boronpenetration in a CMOS or BiCMOS structure, while maintaining the sameelectron channel mobility and keeping the threshold voltage shift to anallowable level. The process of using a torch or pre-combustion furnacechamber for N₂O combustion has been designated “Thermal Nitrogen DopantTuning” (TNDT) and the torch or pre-combustion furnace chamber referredto as the “thermal nitrogen dopant tuner.”

The thermal nitrogen dopant tuner consists of a high temperature heatingelement that can operate in the temperature range of approximately 800°C. to 1200° C. This heating element is used for the pre-combustion ofN₂O outside the main chamber or wafer chamber. The location of theheating element prevents combustion of the N₂O in the wafer processchamber. If combustion of the N₂O is performed in the wafer processchamber, thickness non-uniformities of the oxynitride film will result,due to the exothermic nature of the N₂O gas phase decomposition. Anexothermic reaction will cause NO, O₂, and O to propagate towards theexhaust outlet and across the wafer surface (from the outer wafer radiusto the inner wafer radius) as reactive composition components, bothwithin the wafer and within the load non-uniformities. The temperaturerange of the heating element is critical to the concentration of NO,which is the reactant that incorporates N in the grown film, andincreases as the temperature increases. For temperatures greater that950° C., enough NO₂ bi-product can be produced to react with atomic O,thus decreasing the amount of atomic O compared to increasing thepercent of NO composition in the gas phase. This decreases the amount ofnitrogen (N) removal at the Si/SiOxNy oxynitridation front, causing thedoping concentration at this interface to be increased compared to theconcentration at lower pre-combustion temperatures. This also allows forindependent control of the wafer temperature and gas temperature whileproviding a plentiful source of NO from N₂O to form oxynitride. Theeffect is especially advantageous for device structures which require alow thermal budget.

An example film and device structure have been demonstrated using athermal nitrogen dopant tuner with the desired temperature of 950° C.and a main process tube (main chamber) temperature of 800° C. Electricalthickness uniformities within-wafer (measured using acapacitance-voltage technique) were 0.02 to 0.06 Å. FIG. 1 shows acomparison of capacitance-voltage curves for a thermal nitrogen dopanttuner process 10 and a process using SiO₂ control 12 (without a thermalnitrogen dopant tuner). In generating FIG. 1, sixteen devices weremeasured on both wafers. As indicated by the FIG. 1 curves, excellentuniformity on the order of 0.3 Å or approximately one statisticaldeviation, and small flatband shift on the order of 25 meV, areexhibited by the thermal nitride dopant tuner oxide. Across loadthickness, uniformities were approximately within 0.12 Å for a waferload of one hundred. Nitrogen incorporation in the film was measured atan integrated concentration of 4(10¹⁴) to 5(10¹⁴) atoms/cm². Oxynitridesformed with N₂O under (typical) pre-combustion temperatures of 800° C.to 850° C. yielded nitrogen concentrations of 1(10¹⁴) to 2(10¹⁴)atoms/cm². Oxynitrides formed with NO yielded nitrogen concentrations of6(10¹⁴) to 8(10¹⁴) atoms/cm².

This increase in N incorporation using the thermal nitrogen dopant tunerresulted in a two-fold improvement in leakage current, 0.005 A/cm² (at 1volt reverse bias), versus a leakage current of 0.01 A/cm² without theimplementation of the N₂O thermal nitrogen dopant tuner. FIG. 2represents I-V curves for the thermal nitrogen dopant tuner film 14 andthe SiO₂ control 16. As indicated by these curves, the leakage currentthat resulted from the implementation of the thermal nitrogen dopanttuner is reduced from that produced by the SiO₂ control process. Thisleakage current improvement by a factor of two to three timescorresponds to the gain of the oxide thickness (or capacitance) byapproximately 0.5 to 1 Å. This leakage current is comparable to thatproduced by oxynitride growth using NO as the gas source. However, foran NO gas source, the mobility and transconductance are degraded due totoo much N incorporation. Implementing the N₂O thermal nitrogen dopanttuner exhibited negligible mobility degradation. In addition, theflatband voltage shift was on the order of 20 meV, which remains anacceptably small value.

The leakage current problem has been identified by others skilled inthis art, and solutions have been proposed using thermal growth oranneal to form oxynitride, or N₂ ion implant into the silicon channelwith a subsequent reoxidation to form the nitrogen-rich interfaciallayer. The implementation of the thermal nitrogen dopant tuner of thepresent invention would serve as a replacement of the N₂ ion implantprocess step(s), or in the alternative, enhance its effect on leakagecurrents. Having the thermal nitrogen dopant tuner external to the waferprocess chamber, thickness and dopant non-uniformity defects that occurduring decomposition of N₂O in the wafer chamber are no longer produced.The introduction of this tuner in the process also mitigates thetoxicity problem that results from using NO, by creating a gas streamthat has less NO by-products than pure NO.

The method of forming an insulating layer on a substrate using thethermal nitrogen dopant tuner requires pre-combustion of the N₂O outsidethe main chamber at a temperature higher than that within the mainchamber. Preferably, the main chamber is kept at a temperature in therange of 800° C., while the pre-combustion chamber is in the range of950° C. FIG. 3 is a block diagram representing the gas flow through theseparate chambers used in the gas flow/reaction sequence of the instantinvention. Uncombusted gas is applied to a first chamber 20 where atorch is used to combust chlorine and steam. This gas 22 is thentransported to a pre-combustion chamber 24 for N₂O combustion. Thecombusted gas 26 is then delivered to a main chamber 28 for thegas/wafer reaction where an oxynitride film is formed on the wafer. Thesubstrate remains at a temperature within the main chamber where it isexposed to the nitrogen containing gas from the pre-combustion chamber,delivered at a second temperature which is higher than the temperaturein the main chamber. The resultant insulator is then grown on the waferin the presence of this gas.

A second embodiment of the invention considers using distributed N₂O gasinjectors within the main chamber or process tube. The N₂O gas ispre-heated outside the chamber, as similarly required in the firstembodiment. Next, the N₂O gas is delivered to a gas manifold that splitsthe gas flow and directs the gas to a number of gas injectors,preferably two to four injectors, within the main process tube. Gasinjection orifices on the order of several millimeters then distributethe pre-decomposed gas to the wafers, producing a more uniformly N-dopedwafer load in a batch furnace. This apparatus geometry and method ofimplementation is unique to the application of N-doped SiO₂ oroxynitride.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1. A method of forming an insulating layer on a substrate comprising:heating combusted chlorine and steam gas within a chamber outside afurnace having a main chamber; removing said heated chlorine and steamgas from said chamber; and pre-combustion of nitrogen containing gaswithin said chamber outside said furnace having said main chamber, saidpre-combustion performed at a temperature higher than that within saidmain chamber.
 2. The method of claim 1 wherein said nitrogen containinggas includes N₂O.
 3. The method of claim 1 wherein said nitrogencontaining gas includes NO.
 4. A method of tuning the magnitude ofnitrogen doped profiles for an oxynitride insulator on a substrate, saidmethod comprising: providing said substrate at a first temperaturewithin a main chamber; pre-heating a nitrogen containing gas outsidesaid chamber; transferring said pre-heated nitrogen containing gas tosaid main chamber; exposing said substrate to said nitrogen containinggas at a second temperature which is higher than said first temperature;and growing said insulator on said substrate in the presence of saidgas.
 5. The method of claim 4 wherein said exposing step furthercomprises heating said nitrogen containing gas to said secondtemperature in a second chamber outside said main chamber and directingsaid heated nitrogen containing gas to said main chamber.
 6. A method oftuning the magnitude of nitrogen doped profiles for an oxynitrideinsulator on a substrate, said method comprising: providing saidsubstrate at a first temperature within a main chamber; heating chlorineand steam gas to a combustion temperature and transferring said chlorineand steam gas to a second chamber; exposing said substrate to a nitrogencontaining gas at a second temperature which is higher than said firsttemperature, whereby said nitrogen containing gas is heated to saidsecond temperature in said second chamber outside said main chamber anddirecting said heated nitrogen containing gas to said main chamber; andgrowing said insulator on said substrate in the presence of said gas. 7.The method of claim 6 wherein said heating chlorine and steam gascomprises applying a torch heating element to said chlorine and steamgas in a third chamber separate from said main and second chambers. 8.The method of claim 5 wherein said second temperature is 950° C.
 9. Themethod of claim 5 wherein said first temperature is in the range 600 to1100° C.
 10. The method of claim 5 wherein said first temperature is inthe range 600 to 800° C.
 11. The method of claim 4 wherein said nitrogencontaining gas comprises NO or N₂O.
 12. The method of claim 4 whereinsaid second temperature is sufficient to react with said gas before itreacts with said substrate.
 13. The method of claim 4 wherein saidsecond temperature is in the range of 800 to 1200° C.
 14. The method ofclaim 4 further comprising adjusting said second temperature to tailoran amount of nitrogen concentration in said insulating layer.
 15. Themethod of claim 14 wherein said second temperature is in the range of800 to 1200° C. while the first temperature is in the range of 600 to1100° C.
 16. A method for distributing a nitrogen containing gas to forman insulation film on a semiconductor, comprising: providing saidsemiconductor within a main process tube; pre-heating N₂O gas outsidesaid main process tube; delivering said N₂O gas to a gas manifold;splitting said N₂O gas flow through said gas manifold to direct said N₂Ogas to gas injectors attached to said main process tube; and,distributing said N₂O gas through said injectors to said main processtube housing said semiconductor.
 17. The method of claim 16 whereinpre-heating said N₂O gas comprises raising said N₂O gas to a temperaturewithin the range of 850 to 1200° C. outside said main process tube. 18.The method of claim 16 wherein said insulation film comprises an oxideor oxynitride layer.